Display screen or portion thereof with a graphical user interface



FIG. 1 is a front view of a display screen or portion thereof with a graphical user interface; and,

FIG. 2 is front view of a second embodiment thereof.

The outermost broken lines illustrate a display screen or portion thereof and form no part of the claimed design. The remaining broken lines, illustrating portions of the graphical user interface, form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 